Hydrothermal Alkaline Treatment (HALT) - Revision history

Admin at 20:50, 11 February 2026

2026-02-11T20:50:45Z

Debra Tabron at 16:06, 7 August 2025

2025-08-07T16:06:56Z

Jhurley: /* Reaction Mechanisms and Treatment Efficacy */

2025-08-07T12:19:04Z

‎Reaction Mechanisms and Treatment Efficacy

Jhurley: /* Introduction */

2025-08-07T12:17:50Z

‎Introduction

Jhurley: /* Introduction */

2025-08-07T12:17:15Z

‎Introduction

Jhurley: /* Introduction */

2025-08-04T19:40:21Z

‎Introduction

Jhurley: /* Introduction */

2025-08-04T19:38:51Z

‎Introduction

Jhurley: /* Reaction Mechanisms and Treatment Efficacy */

2025-08-04T19:32:20Z

‎Reaction Mechanisms and Treatment Efficacy

Jhurley: /* Introduction */

2025-08-04T14:42:47Z

‎Introduction

Admin at 01:34, 31 May 2025

2025-05-31T01:34:04Z

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 *[[PFAS Transport and Fate]]
-'''Contributors:''' [[Dr. Brian Pinkard]], [[Dr. Timothy J. Strathmann | Dr. Timothy Strathmann]], and [[Dr. Shilai Hao]]
+'''Contributor(s):''' [[Dr. Brian Pinkard]], [[Dr. Timothy J. Strathmann | Dr. Timothy Strathmann]], and [[Dr. Shilai Hao]]
 '''Key Resource(s):'''

@@ Line 9: / Line 9: @@
 *[[PFAS Transport and Fate]]
-'''Contributors:''' Dr. Brian Pinkard, [[Dr. Timothy J. Strathmann | Dr. Timothy Strathmann]] and Dr. Shilai Hao
+'''Contributors:''' [[Dr. Brian Pinkard]], [[Dr. Timothy J. Strathmann | Dr. Timothy Strathmann]], and [[Dr. Shilai Hao]]
 '''Key Resource(s):'''

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 A mechanistic understanding of the HALT process for PFAS destruction needs further evaluation to optimize the process and reduce the consumption of chemicals and energy. While the studies of neat compounds are relatively straightforward, one of the major challenges is to address the effect of co-contaminants and apply the process to real-world operating scenarios. Recent laboratory studies with batch reactors conducted at the Colorado School of Mines (CSM) have extended the application of HALT for the destruction of PFAS in a variety of contaminated matrices, including groundwater and soils<ref name="HaoEtAl2022"/> and foam fractionation-derived liquid concentrate<ref name="HaoEtAl2023"/>. Apparent rates for the transformation of individual PFAS have been found to be largely insensitive to the type of media<ref name="HaoEtAl2023"/>, but there is a need to account for any reactions with the media that consume OH· (e.g., OH· reactions with silica-containing soil minerals)<ref name="HaoEtAl2022"/> Notably, while alkali is not required to degrade PFCAs, it is still necessary to convert the organically bound fluorine to inorganic F<sup>-</sup>. Austin ''et. al.''<ref name="AustinEtAl2024"/> showed that TFA, a C<sub>1</sub> PFCA, degrades at similar rates in the absence and presence of NaOH, but mineralization to F<sup>-</sup> and CO<sub>3</sub><sup>2-</sup> only occurs when NaOH is added; otherwise [[Wikipedia:Fluoroform | fluoroform (CHF<sub>3</sub>)]] is the terminal product when no NaOH is added to the reaction solution.
-<onlyinclude>HALT can also be applied to destroy other fluorinated compounds, for example, [[Wikipedia:Hydrofluorocarbon | hydrofluorocarbon (HFC)]] refrigerants. HFC refrigerants are known to decompose into PFAS such as TFA in the atmosphere and thereby subsequently appear in concerning concentrations in rainwater.</onlyinclude> By themselves, HFCs are resistant to thermal degradation; however, in the presence of alkali (e.g., NaOH), alkaline hydrolysis can occur at T < 150˚C<ref name="AustinEtAl2024"/>.
+HALT can also be applied to destroy other fluorinated compounds, for example, [[Wikipedia:Hydrofluorocarbon | hydrofluorocarbon (HFC)]] refrigerants. HFC refrigerants are known to decompose into PFAS such as TFA in the atmosphere and thereby subsequently appear in concerning concentrations in rainwater. By themselves, HFCs are resistant to thermal degradation; however, in the presence of alkali (e.g., NaOH), alkaline hydrolysis can occur at T < 150˚C<ref name="AustinEtAl2024"/>.
 ==State of the Art==

@@ Line 25: / Line 25: @@
 <onlyinclude>HALT processing subjects PFAS in an aqueous solution to high pressure, high temperature, and high pH conditions. The required operating conditions are dependent on the specific target PFAS being destroyed, as [[Wikipedia: Perfluoroalkyl carboxylic acids | perfluorocarboxylic acids (PFCAs)]] such as [[Wikipedia: Trifluoroacetic acid |trifluoroacetic acid (TFA)]] can be destroyed under mild conditions</onlyinclude> (e.g., P ~ 2 MPa, T ~ 200 °C, pH ~ 13)<ref name="AustinEtAl2024">Austin, C., Purohit, A., Thomsen, C., Pinkard, B.R., Strathmann, T.J., Novosselov, I.V., 2024. Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA). Environmental Science and Technology, 58(18), pp. 8076-8085.&nbsp; [https://doi.org/10.1021/acs.est.3c09404 doi: 10.1021/acs.est.3c09404]</ref><onlyinclude>, whereas [[Wikipedia: Perfluorosulfonic acids | perfluorosulfonic acids (PFSAs)]] such as [[Wikipedia: Perfluorobutanesulfonic acid | perfluorobutanesulfonic acid (PFBS)]] require more aggressive processing conditions</onlyinclude> (e.g., P ~ 25 MPa, T ~ 350 °C, pH ~ 14.7) [5] (Figure 1)<onlyinclude>. HALT is capable of facilitating complete “mineralization” of PFAS, defined as the conversion of organic fluorine to dissolved inorganic fluoride. </onlyinclude>The treatment time for HALT is relatively shorter (<2 hours) compared to most other PFAS destructive technologies. For instance, treatment of two-fold diluted [[Wikipedia: Firefighting foam | aqueous film-forming foams (AFFFs)]] using HALT in batch mode achieved nearly complete defluorination in just 30 minutes under conditions of 350 °C and 5 M NaOH<ref name="HaoEtAl2021"/>. PFCAs can be destroyed with even faster kinetics at milder conditions; for example, >90% destruction and defluorination of [[Wikipedia: Trifluoroacetic acid | trifluoroacetic acid (TFA)]] was achieved within 40 min at 200 °C<ref name="AustinEtAl2024"/>. Kinetic rate constants for individual PFAS compounds in HALT environments have been proposed in several studies<ref name="AustinEtAl2024"/><ref name="WuEtAl2019">Wu, B., Hao, S., Choi, Y.J., Higgins, C.P., Deeb, R., Strathmann, T.J., 2019. Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction. Environmental Science and Technology Letters, 6(10), pp. 630-636.&nbsp; [https://doi.org/10.1021/acs.estlett.9b00506 doi: 10.1021/acs.estlett.9b00506]</ref>. The fluorine mass balance during HALT processing has also been investigated, showing near-stoichiometric conversion of organic fluorine to inorganic fluoride under optimal conditions<ref name="PinkardEtAl2024a"/>.
-<onlyinclude>From a practical perspective, HALT is best suited for destroying PFAS in concentrated liquids such as liquid concentrate streams produced as byproducts of other water treatment processes (e.g., [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], [[Wikipedia: Foam fractionation | foam fractionation]]). </onlyinclude>Previous publications demonstrate that complex sample matrices, including high concentrations of inorganic salts(e.g., 83 g/L chloride) and dissolved organic carbon (e.g., 13 g/L), do not inhibit the degradation rate of PFAS compared to a clean matrix, such as groundwater<ref name="HaoEtAl2022">Hao, S., Choi, Y.J,. Deeb, R.A., Strathmann, T.J., Higgins, C.P., 2022. Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil. Environmental Science and Technology, 56(10), pp. 6647-6657.&nbsp; [https://doi.org/10.1021/acs.est.2c00654 doi: 10.1021/acs.est.2c00654]</ref><ref name="HaoEtAl2023">Hao, S., Reardon, P.N., Choi, Y.J., Zhang, C., Sanchez, J.M., Higgins, C.P., Strathmann, T.J., 2023. Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater. Environmental Science and Technology 57(44), pp. 17154-17165.&nbsp; [https://doi.org/10.1021/acs.est.3c05140 doi: 10.1021/acs.est.3c05140]</ref>. Moreover, several field demonstrations of HALT have been performed successfully, and the technology is scalable for commercialization.
+<onlyinclude>From a practical perspective, HALT is best suited for destroying PFAS in concentrated liquids such as liquid concentrate streams produced as byproducts of other water treatment processes (e.g., [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], [[Wikipedia: Foam fractionation | foam fractionation]]). </onlyinclude>Previous publications demonstrate that complex sample matrices, including high concentrations of inorganic salts (e.g., 83 g/L chloride) and dissolved organic carbon (e.g., 13 g/L), do not inhibit the degradation rate of PFAS compared to a clean matrix, such as groundwater<ref name="HaoEtAl2022">Hao, S., Choi, Y.J,. Deeb, R.A., Strathmann, T.J., Higgins, C.P., 2022. Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil. Environmental Science and Technology, 56(10), pp. 6647-6657.&nbsp; [https://doi.org/10.1021/acs.est.2c00654 doi: 10.1021/acs.est.2c00654]</ref><ref name="HaoEtAl2023">Hao, S., Reardon, P.N., Choi, Y.J., Zhang, C., Sanchez, J.M., Higgins, C.P., Strathmann, T.J., 2023. Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater. Environmental Science and Technology 57(44), pp. 17154-17165.&nbsp; [https://doi.org/10.1021/acs.est.3c05140 doi: 10.1021/acs.est.3c05140]</ref>. Moreover, several field demonstrations of HALT have been performed successfully, and the technology is scalable for commercialization.
 ==Reaction Mechanisms and Treatment Efficacy==

@@ Line 25: / Line 25: @@
 <onlyinclude>HALT processing subjects PFAS in an aqueous solution to high pressure, high temperature, and high pH conditions. The required operating conditions are dependent on the specific target PFAS being destroyed, as [[Wikipedia: Perfluoroalkyl carboxylic acids | perfluorocarboxylic acids (PFCAs)]] such as [[Wikipedia: Trifluoroacetic acid |trifluoroacetic acid (TFA)]] can be destroyed under mild conditions</onlyinclude> (e.g., P ~ 2 MPa, T ~ 200 °C, pH ~ 13)<ref name="AustinEtAl2024">Austin, C., Purohit, A., Thomsen, C., Pinkard, B.R., Strathmann, T.J., Novosselov, I.V., 2024. Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA). Environmental Science and Technology, 58(18), pp. 8076-8085.&nbsp; [https://doi.org/10.1021/acs.est.3c09404 doi: 10.1021/acs.est.3c09404]</ref><onlyinclude>, whereas [[Wikipedia: Perfluorosulfonic acids | perfluorosulfonic acids (PFSAs)]] such as [[Wikipedia: Perfluorobutanesulfonic acid | perfluorobutanesulfonic acid (PFBS)]] require more aggressive processing conditions</onlyinclude> (e.g., P ~ 25 MPa, T ~ 350 °C, pH ~ 14.7) [5] (Figure 1)<onlyinclude>. HALT is capable of facilitating complete “mineralization” of PFAS, defined as the conversion of organic fluorine to dissolved inorganic fluoride. </onlyinclude>The treatment time for HALT is relatively shorter (<2 hours) compared to most other PFAS destructive technologies. For instance, treatment of two-fold diluted [[Wikipedia: Firefighting foam | aqueous film-forming foams (AFFFs)]] using HALT in batch mode achieved nearly complete defluorination in just 30 minutes under conditions of 350 °C and 5 M NaOH<ref name="HaoEtAl2021"/>. PFCAs can be destroyed with even faster kinetics at milder conditions; for example, >90% destruction and defluorination of [[Wikipedia: Trifluoroacetic acid | trifluoroacetic acid (TFA)]] was achieved within 40 min at 200 °C<ref name="AustinEtAl2024"/>. Kinetic rate constants for individual PFAS compounds in HALT environments have been proposed in several studies<ref name="AustinEtAl2024"/><ref name="WuEtAl2019">Wu, B., Hao, S., Choi, Y.J., Higgins, C.P., Deeb, R., Strathmann, T.J., 2019. Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction. Environmental Science and Technology Letters, 6(10), pp. 630-636.&nbsp; [https://doi.org/10.1021/acs.estlett.9b00506 doi: 10.1021/acs.estlett.9b00506]</ref>. The fluorine mass balance during HALT processing has also been investigated, showing near-stoichiometric conversion of organic fluorine to inorganic fluoride under optimal conditions<ref name="PinkardEtAl2024a"/>.
-<onlyinclude>From a practical perspective, HALT is best suited for destroying PFAS in concentrated liquids such as liquid concentrate streams produced as byproducts of other water treatment processes (e.g., [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], [[Wikipedia: Foam fractionation | foam fractionation]]). Previous publications demonstrate that complex sample matrices, including high concentrations of inorganic salts</onlyinclude>(e.g., 83 g/L chloride)<onlyinclude> and dissolved organic carbon</onlyinclude> (e.g., 13 g/L)<onlyinclude>, do not inhibit the degradation rate of PFAS compared to a clean matrix, such as groundwater</onlyinclude><ref name="HaoEtAl2022">Hao, S., Choi, Y.J,. Deeb, R.A., Strathmann, T.J., Higgins, C.P., 2022. Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil. Environmental Science and Technology, 56(10), pp. 6647-6657.&nbsp; [https://doi.org/10.1021/acs.est.2c00654 doi: 10.1021/acs.est.2c00654]</ref><ref name="HaoEtAl2023">Hao, S., Reardon, P.N., Choi, Y.J., Zhang, C., Sanchez, J.M., Higgins, C.P., Strathmann, T.J., 2023. Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater. Environmental Science and Technology 57(44), pp. 17154-17165.&nbsp; [https://doi.org/10.1021/acs.est.3c05140 doi: 10.1021/acs.est.3c05140]</ref><onlyinclude>. Moreover, several field demonstrations of HALT have been performed successfully, and the technology is scalable for commercialization. </onlyinclude>
+<onlyinclude>From a practical perspective, HALT is best suited for destroying PFAS in concentrated liquids such as liquid concentrate streams produced as byproducts of other water treatment processes (e.g., [[PFAS Treatment by Anion Exchange | regenerable ion exchange]], [[Wikipedia: Foam fractionation | foam fractionation]]). </onlyinclude>Previous publications demonstrate that complex sample matrices, including high concentrations of inorganic salts(e.g., 83 g/L chloride) and dissolved organic carbon (e.g., 13 g/L), do not inhibit the degradation rate of PFAS compared to a clean matrix, such as groundwater<ref name="HaoEtAl2022">Hao, S., Choi, Y.J,. Deeb, R.A., Strathmann, T.J., Higgins, C.P., 2022. Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil. Environmental Science and Technology, 56(10), pp. 6647-6657.&nbsp; [https://doi.org/10.1021/acs.est.2c00654 doi: 10.1021/acs.est.2c00654]</ref><ref name="HaoEtAl2023">Hao, S., Reardon, P.N., Choi, Y.J., Zhang, C., Sanchez, J.M., Higgins, C.P., Strathmann, T.J., 2023. Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater. Environmental Science and Technology 57(44), pp. 17154-17165.&nbsp; [https://doi.org/10.1021/acs.est.3c05140 doi: 10.1021/acs.est.3c05140]</ref>. Moreover, several field demonstrations of HALT have been performed successfully, and the technology is scalable for commercialization.
 ==Reaction Mechanisms and Treatment Efficacy==

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 ==Introduction==
 [[File:PinkardFig1.png|thumb|350px|left|Figure 1. HALT refers to the [[Wikipedia: Critical_point_(thermodynamics)#Liquid–vapor critical point | subcritical]] water region on the pressure–temperature phase diagram of water]]
-<onlyinclude>Hydrothermal alkaline treatment (HALT) is a thermochemical processing technology effective at destroying and defluorinating halogenated organic compounds such as [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) ||per- and polyfluoroalkyl substances (PFAS)]]. </onlyinclude>HALT is also known as “[[Wikipedia: Hydrolysis#Alkaline_hydrolysis |alkaline hydrolysis]],” and is very similar to processing technologies such as [[Wikipedia: Hydrothermal liquefaction | hydrothermal liquefaction (HTL)]] which have been developed and investigated for organic waste-to-energy applications.
+<onlyinclude>Hydrothermal alkaline treatment (HALT) is a thermochemical processing technology effective at destroying and defluorinating halogenated organic compounds such as [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | per- and polyfluoroalkyl substances (PFAS)]]. </onlyinclude>HALT is also known as “[[Wikipedia: Hydrolysis#Alkaline_hydrolysis |alkaline hydrolysis]],” and is very similar to processing technologies such as [[Wikipedia: Hydrothermal liquefaction | hydrothermal liquefaction (HTL)]] which have been developed and investigated for organic waste-to-energy applications.
 <onlyinclude>HALT processing subjects PFAS in an aqueous solution to high pressure, high temperature, and high pH conditions. The required operating conditions are dependent on the specific target PFAS being destroyed, as [[Wikipedia: Perfluoroalkyl carboxylic acids | perfluorocarboxylic acids (PFCAs)]] such as [[Wikipedia: Trifluoroacetic acid |trifluoroacetic acid (TFA)]] can be destroyed under mild conditions</onlyinclude> (e.g., P ~ 2 MPa, T ~ 200 °C, pH ~ 13)<ref name="AustinEtAl2024">Austin, C., Purohit, A., Thomsen, C., Pinkard, B.R., Strathmann, T.J., Novosselov, I.V., 2024. Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA). Environmental Science and Technology, 58(18), pp. 8076-8085.&nbsp; [https://doi.org/10.1021/acs.est.3c09404 doi: 10.1021/acs.est.3c09404]</ref><onlyinclude>, whereas [[Wikipedia: Perfluorosulfonic acids | perfluorosulfonic acids (PFSAs)]] such as [[Wikipedia: Perfluorobutanesulfonic acid | perfluorobutanesulfonic acid (PFBS)]] require more aggressive processing conditions</onlyinclude> (e.g., P ~ 25 MPa, T ~ 350 °C, pH ~ 14.7) [5] (Figure 1)<onlyinclude>. HALT is capable of facilitating complete “mineralization” of PFAS, defined as the conversion of organic fluorine to dissolved inorganic fluoride. </onlyinclude>The treatment time for HALT is relatively shorter (<2 hours) compared to most other PFAS destructive technologies. For instance, treatment of two-fold diluted [[Wikipedia: Firefighting foam | aqueous film-forming foams (AFFFs)]] using HALT in batch mode achieved nearly complete defluorination in just 30 minutes under conditions of 350 °C and 5 M NaOH<ref name="HaoEtAl2021"/>. PFCAs can be destroyed with even faster kinetics at milder conditions; for example, >90% destruction and defluorination of [[Wikipedia: Trifluoroacetic acid | trifluoroacetic acid (TFA)]] was achieved within 40 min at 200 °C<ref name="AustinEtAl2024"/>. Kinetic rate constants for individual PFAS compounds in HALT environments have been proposed in several studies<ref name="AustinEtAl2024"/><ref name="WuEtAl2019">Wu, B., Hao, S., Choi, Y.J., Higgins, C.P., Deeb, R., Strathmann, T.J., 2019. Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction. Environmental Science and Technology Letters, 6(10), pp. 630-636.&nbsp; [https://doi.org/10.1021/acs.estlett.9b00506 doi: 10.1021/acs.estlett.9b00506]</ref>. The fluorine mass balance during HALT processing has also been investigated, showing near-stoichiometric conversion of organic fluorine to inorganic fluoride under optimal conditions<ref name="PinkardEtAl2024a"/>.